1. Field of the Invention
The invention relates to an electronic camera for forming images by mechanically scanning an imaging device.
2. Description of the Prior Art
Imaging methods used by conventional still-camera type image input devices include the common method using a two-dimensional imaging device (commonly called an "area sensor"), and methods whereby a two-dimensional image is obtained by mechanically scanning an image using a one-dimensional imaging device (commonly called a "line sensor") as disclosed in Japanese laid-open (tokkai) patent number H5-316302.
An area sensor 60 with an imaging area of H pixels by V pixels is shown in FIG. 11, and a V-pixel long line sensor 61 which is driven in the direction of the arrow to subscan H pixels wide to image an area of H pixels by V pixels is shown in FIG. 12. If the per-pixel sensitivity of these sensors is equal, the line sensor requires an imaging time of H times that of the area sensor due to the charge storage time in order to achieve the same sensitivity. On the other hand, the line sensor has 1/H as many pixels as the area sensor, can therefore be achieved with a smaller sensor chip, and is thus used in image input devices having a large pixel count.
In one specialized image input method an image moving continuously across an area sensor is obtained by a time-delay integration (TDI) operation. Examples of devices using this TDI operation are disclosed in U.S. Pat. No. 4,740,681, an apparatus for capturing images of stars that are moving due to the rotation of the imaging satellite, and U.S. Pat. No. 4,922,337, an apparatus for capturing images of objects moving at a constant rate on a conveyor as shown in FIG. 13.
Referring to FIG. 13, a subject 38 on a conveyor 37 moving at a constant rate of speed is imaged by a camera 36 comprising a lens 35 and an area sensor 31. The area sensor 31 executes a TDI operation synchronized to the output of a tachometer 39 detecting the speed of the conveyor 37 to capture a still image of the subject 38.
What is common to each of the above-described apparatuses is that images are captured of subjects moving at a constant speed in a constant direction relative to a camera comprising a sensor that is in a fixed position relative to the lens. There are no examples in the literature of methods using this TDI operation compatible with conventional cameras used to capture images of common, indeterminately-moving subjects.
Using an area sensor to construct a high resolution camera such as an HDTV camera having more than two-million pixels requires a large-scale chip, resulting in an extremely high cost. Camera-type scanners using a line sensor are widely available, but require a per-image scanning time of many seconds. As a result such scanners are limited to applications in which the camera is fixed on a stand and used to capture still subjects.